Document 2RnwRNpZ1Jq0Z62JgoY9Ba0wN

642 JEROME R. COX, JR. coefficient, a (alpha). An open window has an absorption coefficient of 1, || a piece of marble has an absorption coefficient very nearly zero. The absorption coefficient, ce, of most materials is not the same forTM quencies. This is especially true of the "acoustical materials," which are'cf for high absorption. Figure 12 shows the absorption coefficient for sever!! 1.0 (a) sW (4)/" (e) ^ e 8 4 0.2 (> / (t)/ ..<S ^ -m\ Frequency (CPS) Figure 12. The absorption coefficient of various materials (a reverberant soiind! assumed): (a) 4-inoh glass wool blanket (6 Ib./cu. ft. density) against wall--nostfaffi average for perforated acoustic tiles cemented directly to hard backing and 1 inchl(tlv| average for perforated acoustic tiles cemented directly to hard backing and `/t it" (d) 1-inch mineral wool blanket (Aerocor--2 lb./cu. ft.) covered by perforated facing! cent open area) and separated from hard backing by 1-inch air space;* (e) VrinclT panel with 4-inch air space, one cross brace, and 2 inches of mineral wool at walt;tjl carpet on concrete ;* (p) unpainted brick wall." rials in the audible frequency range. Published tables are available giving rlaia other materials^5-7 Thereis no single ideal absorbing material: one-mfeli>**>' with characteristics that are best for the particular problem at hand. The average absorption coefficient, a, of a room is a good intiofJoS *U. Ingard and R. H. Bolt, Absorption characteristics of acoustic material with pel facingB, J. Acouat. Soc. Am., 23, 533 (1951). Ijjiji ---------AE,JUtarkin-and-H-rJ,-RurkisrSound-ab9orption-of-woad-pane:lff'ft)Ftlti_RByE!|l Hall, Acusttca, 1, 81 (1951). 1 *1 *B. C. Purcell, private communication to the author. V. 0. Knudsen and C. M. Harris, Acoustical Designing in Architecture. Wile York, 1950. * Sound Absorption Coefficients of Architectural Acoustical Materials. Aooustical'tfjij rials Association, 206 W. Monroe St., Chicago, 111. (published periodically). tg' T Sound absorption coefficients of the more common acoustical materials, Nath Standards Bull., Letter Circular LC 870. ' NOISE AND THE CONSERVATION OF HEARING 643 sound field within that room. One may calculate a by multiplying *afiifeaterial in the room by the total number of square feet of wall surface 5j^ped by that material, adding all such products, and dividing the total fastin'ace area of the room. This computation is shown in equation 5. ?ZfiTVrr / -- __Slttl -j- iSsOts -f- `S,3Q;3 -fSi + + jSs-|- (5) tp'oc, 3, and so forth, are the absorption coefficients of the various materials ijSronmand Si, S2, Sa, and so forth, are the surface areas of these materials, Naturally, the entire surface of the room including ceiling and floor Accounted for in this calculation. Sjlan example, determine the average absorption coefficient, 3, at 600 c.p.s., room with 1-inch acoustical tile on the ceiling (a = 0.95), brick walls . 'wu jbBAlW V and a carpet on the floor (a = 0.37). Since each side of the cube has ijcqual to Ve of the total area, the computation is as follows: _ 0.951s -}- 0.371s + 0.03 (41s) :0.24 61s the length of a side of the cube. Efili if- F. SOUND IN A ROOM WITH ABSORBING WALLS "absorbing material is placed on the walls of a room, less sound is re;the room after each reflection. The sound that is absorbed by the wall foto heat and can never return to ..the room. If the walls of the pPperfectly absorbing the situation would be exactly the same as that "Figure 10 because there would be no reflected sound. If, however, some ction is returned to the room after each reflection, the sound field will be rate between the free field (Figure 10) and the reverberant field (Figure imple of this intermediate situation is shown in Figure 13. The average non coefficient that has been assumed for this example is 0.9. vtes-. tr 43: A graphical representation of the radiation of sound from a simple source in a room with absorbing walls.